The seed is a complex, well-protected package
A seed contains tissues from three generations (Figure 28.5). A seed coat develops from the integument—the tissues of the diploid sporophyte parent that surround the megasporangium. Within the megasporangium is haploid tissue from the female gametophyte, which contains a supply of nutrients for the developing embryo. (This tissue is fairly extensive in most gymnosperm seeds. In angiosperm seeds it is greatly reduced, and nutrition for the embryo is supplied instead by a tissue called endosperm.) In the center of the seed is the third generation, the embryo of the new diploid sporophyte.
Figure 28.5 A Seed Develops These cross sections diagram the development of the ovule into a seed in a gymnosperm (Pinus sp.). Angiosperm seed development has differences (e.g., angiosperm integuments have two layers rather than one, and the angiosperm embryo is nourished by specialized tissue called endosperm) but follows the same principle (compare Figures 28.8 and 28.16). (A) The haploid megaspore is nourished by tissues of the parental sporophyte (the diploid megasporangium). (B) The mature megaspore is fertilized by a pollen grain that penetrates the integument, germinates (grows a pollen tube; see Figure 28.4A), and releases a sperm nucleus. (C) Fertilization initiates production of a seed. A mature seed contains three generations: a diploid embryo (the new sporophyte), which is surrounded by haploid female gametophyte tissue that supplies nutrition, which is in turn surrounded by the seed coat (diploid parental sporophyte tissue).
investigating life
William Beal’s Seed Viability Study
experiment
Original Papers: Beal, W. J. 1884. The vitality of seeds. Proceedings of the Society for the Promotion of Agricultural Sciences 5: 44–46.
Darlington, H. T. 1941. The sixty-year period for Dr. Beal’s seed viability. American Journal of Botany 28: 271–273.
Kivilaan, A. and R. S. Bandurski. 1981. The one hundred-year period for Dr. Beal’s seed viability experiment. American Journal of Botany 68: 1290–1292.
Telewski, F. W. and J. Zeevaart. 2002. The 120th year of the Beal seed viability study. American Journal of Botany 89: 1285–1288.
William Beal began an experiment in 1879 to measure the long-term viability of seeds of several common plants. This ongoing experiment has been continued by biologists for well over a century. For the first 40 years of the experiment, Beal checked seed viability every 5 years. H. T. Darlington took over the experiment in 1915 and extended the sampling period to 10-year intervals beginning in 1920. R. S. Bandurski took over the experiment when Darlington retired, and extended the sampling period to 20 years in 1980, a century after the experiment began. Results for three species of plants in years 50–100 of the experiment are shown here.
work with the data
Use the data presented in the preceding experiment to answer the following questions.
Question
1
Calculate the percent of viable seeds for these three species in years 50–100 and graph seed survivorship as a function of time buried.
Question
2
No seeds of the first two species were viable after 90 years of the experiment. Assume 100 percent seed viability at the start of the experiment (year 0), and predict from your graph the approximate year when you think the last of the Verbascum blattaria seeds will germinate.
One approach to this problem is to calculate a linear trend line for survivorship of Verbascum blattaria seeds by calculating a linear regression line (see Appendix B) and then projecting it forward in time to the point at which it intersects zero percent survival. The resulting regression equation is y = 102.09 – 0.62x. The graph plotted in answering Question 1 uses this approach and predicts that the last Verbascum blattaria seeds should germinate in about year 165 of the experiment (set y to 0, and solve for x using the regression equation; the result is x = 164.7 years). This approach assumes a linear decline in viability of the seeds. It may be more reasonable to assume an exponential decay in seed viability (similar to radioactive decay; see Figure 24.1). If seeds decay exponentially, then we would expect some low level of survivorship of Verbascum blattaria seeds well beyond year 165.
Question
3
What factors do you think might influence the differences among the species in long-term seed viability?
At least four factors are related to seed survivorship:
- Size of the seed: larger seeds have more food reserves (endosperm).
- Thickness of the seed coat: thicker seed coats provide better protection of the seed.
- Density of the seed coat: tougher seed coats provide better protection of the seed.
- Level of dormancy of the embryos: deeper dormancy results in longer survivorship.
A similar work with the data exercise may be assigned in LaunchPad.
The seed is a well-protected resting stage. As we discussed in the opening of this chapter, the seeds of some species may remain dormant but stay viable (capable of growth and development) for many years, germinating only when conditions are favorable for the growth of the sporophyte. During the dormant stage, the seed coat protects the embryo from excessive drying and may also protect it against potential predators that would otherwise consume the embryo and its nutrient reserves. Many seed plants have structural adaptations that promote the dispersal of seeds by wind, water, or by animals. When the young sporophyte resumes growth, it draws on the food reserves in the seed. The possession of seeds is a major reason for the enormous evolutionary success of the seed plants, which are the dominant life forms of most modern terrestrial floras.
The germination of the Judean date described in the opening story is an extreme example of seed dormancy. How do we know how long most seeds remain viable? To find out, William J. Beal, a biologist at Michigan State University, decided to begin an experiment in 1879 that he could not hope to finish in his lifetime (Investigating Life: William Beal’s Seed Viability Study). He prepared 20 lots of seeds for long-term storage. Each lot consisted of 50 seeds from each of 23 species of plants. He mixed each lot of seeds with sand and placed the mixture in an uncapped bottle, then buried all the bottles upside down (so they would stay dry) on a sandy knoll. The seeds experienced normal temperature fluctuations for Michigan. At regular intervals ever since, other biologists have excavated a bottle and checked the viability of the seeds it contained. The seeds of most species remained viable for decades, whereas others have remained viable for more than a century.
